where N is the total stat budget you hvae for shield+defense+absorb (in rating).

You have the wrong expression for the FL/TkT heal. Specifically, that ability ticks four times for a total of 8% of max HP. Correcting that expression yields a result of 237.233 with the heal proc relic and 249.367 for the added mitigation budget. 2000 is about the mean DPS for HM EC/TfB, so that's a pretty good benchmark.

However, there are a couple things you need to take into account. Your squishiness result is far, far lower than I can achieve. At a stat budget of 1526 and using the proc absorb relic, my optimized weighted squishiness is 0.3693. That is using the following weighting:

m/r+k/e : 71.80%

f/t+k/e : 18.78%

f/t+i/e : 9.41%

Thus, I strongly suspect that your formulae are inflating the value of mitigation stats by quite a bit. You are definitely weighting defense more highly than I do (by factoring in accuracy as a straight addition to the defense chance), but I need to do some serious combat log perusal before I can conclude anything about that. My formulae may simply be wrong here.

In any case, the conclusion stands that the heal proc relic is still (currently) better than an additional 100 stat points. Once bosses reach a damage value sufficient to flip the numbers, we can reevaluate. At that point though, we will have bigger problems with the self-heal as a general mechanic.

I don't have full damage numbers yet for NiM EC, but preliminary results suggest that damage was buffed by about 15%. That puts average pre-mitigation boss DPS at 2300 in NiM EC/TfB, which is still *far* below the tipping point on the self heal relic. It is worth noting that NiM KP is already above the tipping point, with an average DPS of 3991. I don't consider that op to be current content, and we already knew that shadows/assassins had balance issues when tanking those bosses.